1. Field of the Invention
The present invention relates to a method of forming a thin film of an inorganic solid electrolyte. In particular, the invention relates to a method of forming a thin film of an inorganic solid electrolyte applicable to an electrode of a lithium cell.
2. Description of the Background Art
A solid secondary cell with a thin lithium film has been proposed. Japanese Patent Laying-Open No. 62-44960 discloses a method for manufacturing such a solid cell. The method includes a process of successively forming a thin film of titanium disulfide as a positive electrode, a thin film of Li2Oxe2x80x94Al2O3 as an electrolyte, and a thin film of Li as a negative electrode on a substrate placed in an ionized cluster beam evaporation system.
Japanese Patent Publication No. 5-48582 discloses an electrolytic material for solid cells. The electrolytic material has a composition of aXxe2x80x94bLi2Sxe2x80x94Y wherein X is selected from the group consisting of P2S5 and SiS2, a is in the range of approximately 0.5 to approximately 2, b is in the range of 0.25 to 2, and Y is at least one type of oxygen-containing lithium compound. The composite material has an ionic conductivity of at least 0.75xc3x9710xe2x88x924 ohmxe2x88x921 cmxe2x88x921 at 25xc2x0 C. In this conventional technique, the electrolytic material is produced through the fusion of the source materials and quenching of the fused materials.
On the other hand, advances have been made in commercialization of lithium secondary cells containing an organic solution of electrolytes. Lithium secondary cells are characterized by having a high-energy output per unit volume or per unit weight as compared with other cells. Lithium secondary cells have been developed as a power source for practical use in mobile communications equipment, notebook computers, electric vehicles and the like.
An attempt has been made to use lithium metal for a negative electrode for the purpose of improving the performance of the lithium secondary cell. Such a lithium negative electrode, however, has been accompanied by the risk of a dendroid growth of the lithium metal on the negative electrode during charging and discharging. The dendroid growth may form an internal short-circuit to the positive electrode and finally result in an explosion. An investigated technique for avoiding the risk is to form a thin film of a sulfide-based inorganic solid electrolyte on the lithium metal. Such a technique, however, has been accompanied by a problem that the thin film of the sulfide-based inorganic solid electrolyte formed on a base member through vapor deposition does not exhibit a significantly high ionic conductance.
U.S. Pat. No. 6,025,094 discloses a method for protecting the negative electrode of lithium from the above-described dendroid lithium metal by covering one surface of the negative electrode of lithium with a glassy or amorphous protection layer. The protection layer is made of 6LiIxe2x80x94Li3PO4xe2x80x94P2S5, B2O3xe2x80x94LiCO3xe2x80x94Li3PO4, LiIxe2x80x94Li2Oxe2x80x94SiO2 or LixPOyNz, for example, and formed on the lithium metal electrode by a plasma assisted deposition technique. The U.S. patent discloses that this protection layer conducts lithium ions. However, such a protection layer formed by the plasma assisted deposition technique may also have the problem that it does not exhibit a significantly high ionic conductivity. The U.S. Patent does not disclose or suggest any technique for enhancing the ion conductivity of the protection layer.
Recently, it is reported in xe2x80x9cThe 26th Symposium on Solid State Ionics in Japan, November, 2000, Extended Abstract, pages 174 and 175xe2x80x9d that fast lithium ion conducting glass-ceramics were synthesized by heat treatment at around 200xc2x0 C. of Li2Sxe2x80x94P2S5 amorphous powders. However, these glass-ceramics were bulky material in which Li7PS6 crystal phase was mainly precipitated.
One object of the present invention is to provide a method of producing a thin film of an inorganic solid electrolyte having a relatively high ionic conductance.
The inventors of the present invention have found that the ionic conductance of the thin film made of an inorganic solid electrolyte can be enhanced by forming the thin film of the inorganic solid electrolyte on a base member while heating the base member, or by forming the thin film of the inorganic solid electrolyte on the base member and thereafter heating the thin film.
Accordingly, the present invention is directed to a method of forming a thin film made of an inorganic solid electrolyte on a base member. The inventive method includes the step of forming, by a vapor deposition method, the thin film made of the inorganic solid electrolyte on the base member while the base member is being heated so that the thin film is caused to have an ionic conductance higher than that of such a thin film formed on a base member without being heated.
The present invention is directed to another method of forming a thin film made of an inorganic solid electrolyte on a base member. The inventive method includes the steps of forming the thin film made of the inorganic solid electrolyte on the base member at room temperature or at a temperature lower than 40xc2x0 C., and heating the thin film made of the inorganic solid electrolyte to increase the ionic conductance of the thin film.
In the inventive methods, the temperature of the base member being heated is preferably 40xc2x0 C. or higher and lower than the glass transition temperature of the thin film made of the inorganic solid electrolyte. In particular, the temperature of the base member being heated is preferably 40xc2x0 C. to 200xc2x0 C. and more preferably 100xc2x0 C. or higher and lower than 179xc2x0 C.
In the inventive methods, preferably, the thin film made of the inorganic solid electrolyte is heated at a temperature of 40xc2x0 C. or higher and lower than the glass transition temperature of the thin film made of the inorganic solid electrolyte to increase the ionic conductance of the thin film. In particular, the thin film made of the inorganic solid electrolyte is heated preferably at a temperature of 40xc2x0 C. to 200xc2x0 C., more preferably at a temperature of 100xc2x0 C. or higher and lower than 179xc2x0 C., to increase the ionic conductance of the thin film.
In the inventive methods, the inorganic solid electrolyte preferably is a sulfide. In particular, the inorganic solid electrolyte preferably contains lithium with its content of 20% to 65% by atomic percent, one or more elements selected from the group consisting of phosphorus, silicon, boron, germanium and gallium, and sulfur. The inorganic solid electrolyte may further contain at least one element selected from the group consisting of oxygen and nitrogen.
In the methods according to the present invention, the finally produced thin film can have an ionic conductance higher than 5xc3x9710xe2x88x924S/cm. In the method according to the present invention, the finally produced thin film can have an activation energy of 40 kJ/mol or lower.
In the methods according to the present invention, the thin film preferably has a thickness of 0.01 xcexcm to 10 xcexcm.
The base member used for the present invention may have a surface made of at least one type of metal selected from the group consisting of lithium and lithium alloy. The thin film can be formed on the surface made of the metal. In this case, the base member can be used for a lithium cell.
In the present invention, the vapor deposition method is any one method selected from the group consisting of sputtering, vacuum evaporation, laser ablation and ion plating.
The thin film finally obtained in the present invention is typically amorphous or glassy.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.